esptool_py defines command `esptool_py_flash_project_args` that
generates arg file for esptool.py. Two of the arguments are the offset
and image, which are not being used when a template file is given.
This commit makes variables OFFSET and IMAGE available to the template
file, which will holds the value of the offset and image arguments to
`esptool_py_flash_project_args`.
Since OUTPUT argument of custom command does not currently support
generator expressions, the project image is only generated as a side
effect. The primary generated file is a timestamp file. Unfortunately as a consequence
the output logs when the
binary is about to be generated is not as helpful anymore.
Set a custom comment that is more descriptive of what is happening,
and provide more feedback as to what has been generated.
A problem if the Python interpreter used for idf.py (or set via PYTHON
variable) didn't match
"/usr/bin/env python" (or the associated executable for .py files, on
Windows).
Closes https://github.com/espressif/esp-idf/issues/3160
Possibly also fix for https://github.com/espressif/esp-idf/issues/2936
Adds build system test to catch any future direct execution of Python in
the standard build process.
